1. Field of the Invention
The present invention relates to a technology of bending a metal plate, such as a plate-shaped metal workpiece (plate-shaped workpiece) used as an outer-plate (or a skin) of an aircraft, into an arc shape (cylindrical shape) at a predetermined curvature.
2. Description of the Related Art
Hitherto, an outer shell of an aircraft (having a substantially cylindrical shape in horizontal cross-section) is obtained by coupling together several outer-plates (skins) of the aircraft each bent at a predetermined curvature (for example, each having approximately a size of a thickness of from 2 mm to 10 mm×a width of 2.5 m×a length of from 6 m to 10 m) so as to be formed into a cylindrical shape. Accordingly, outer-plate (or skin) workpieces (plate-shaped workpieces) each cut into a predetermined size are subjected to tip forming by a large-sized press brake (forming machine) one by one (three-point bending (see FIG. 8) is performed repeatedly at intervals of approximately 20 mm about one hundred and twenty five times, or three-point bending is performed more times when adjustment of a curvature is necessary), thereby forming each of the outer-plate workpieces (by cylindrical bending or constant curvature bending) into a single contour having a curvature radius R of approximately 3,000 mm. Then, the plurality of outer-plate workpieces (plate-shaped workpieces) each formed to have the predetermined curvature are coupled together, thereby obtaining the outer shell of the aircraft.
In this case, in order to reduce a weight of the aircraft, a plurality of pocket grooves (dents) are formed on an inner side (side to be punched) of the outer-plate of the aircraft, which is to be subjected to cylindrical bending. There are actually a variety of shape patterns of the pocket grooves (dents) (see reference symbol 3A in FIG. 1B, and FIG. 8).
When a region having the pocket grooves (dents) formed therein is subjected to cylindrical bending, as illustrated in FIG. 8, a board (such as a cardboard (filler having a hardness nearly equal to a hardness of a plate)), which conforms to a size and a shape of each of the pocket grooves, is placed in advance (embedded or fitted) in each of the pocket grooves (for example, see Japanese Patent Application Laid-open No. 2012-213792 and Japanese Patent Application Laid-open No. 2011-194426), and forming is performed under a state in which concaves and convexes are eliminated (a thickness of an entire region of the outer-plate workpiece is equalized). In this manner, a product formed into a uniform contour (product bent into a cylindrical shape with a predetermined curvature) is obtained.
In this case, in order to obtain a predetermined contour (curvature or profile), every time pressing operation is performed several times, an operator needs to measure the contour, and to minutely adjust a pushing amount of a punch.
It is necessary to minutely adjust a thickness of the cardboard (filler) in order to minutely adjust the pushing amount of the punch. Operation itself of embedding the cardboard into each of the pocket grooves is complicated and requires a long period of time. In addition, even a skilled worker spends a long period of time to minutely adjust the thickness of the cardboard (minutely adjust a height thereof). Accordingly, in actuality, it takes, for example, four hours to form and complete one outer-plate.
In addition, the outer-plate workpiece (plate-shaped workpiece) of the aircraft is a significantly large component having a width dimension of approximately 2.5 m and a length dimension (longitudinal dimension) of approximately from 6 m to 10 m, despite of a small thickness of approximately from 2 mm to 10 mm. Therefore, the outer-plate workpiece deflects due to a self-weight after forming. Accordingly, when the contour is measured in the deflecting state, in actuality, it is difficult to conduct with good accuracy an inspection of whether or not the predetermined curvature is obtained, and a skill is required.
Further, in order to prevent damage to the outer-plate workpiece at the time of forming, rubber is wound around the punch, and a cover plate (which is a member softer than the outer-plate workpiece) is laid on a die. Accordingly, forming accuracy changes due to aged deterioration of the rubber and the cover plate. Thus, in actuality, it is difficult to perform forming at fixed accuracy for a long period of time. Further, in a tip forming method, a distance between front and rear bearing points of the die (see FIG. 8) is small. As a result, a load applied to deform the workpiece is increased, and a press is required to have high capacity.
Further, hitherto, the outer-plate (or skin) has been formed by the tip forming method using the press brake (forming machine). Accordingly, in order to receive the outer-plate workpiece having the length dimension (longitudinal dimension) of approximately from 6 m to 10 m, a large-sized press brake having a frontage (column interval) of 10 m or more has been needed.
Accordingly, as illustrated in FIG. 7A and FIG. 7B, a slide (punch) of the press brake deflects, and hence has an immense size in order to ensure rigidity (to suppress longitudinal deflection), which leads to increase of mass and increase of operation energy. In addition, a long period of time is needed to store the operation energy, and a long operation cycle is needed. Thus, in actuality, production efficiency of the press brake is low.
Adoption of a three-point press forming method (FIG. 8) employing the tip forming method when bending the outer-plate of the aircraft into a cylindrical shape is considered as a cause of the following: work that has hitherto required a long period of working hours (work of embedding cardboards into the pocket grooves (dents) and work of forming the outer-plate in while minutely adjusting thicknesses of the cardboards); and the press brake (forming machine) having an immense size and a wide frontage for receiving a large workpiece.
In this context, it is desired to create a new forming method that needs no work of filling the pocket grooves even when any patterns of pocket grooves are formed, is capable of obtaining a product shape in several forming steps, and needs no large-sized forming machine.